The low number contradicts statements by Trump administration officials

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Migrants looks for a place to jump the border fence to get into the U.S. side to San Diego, Calif., from Tijuana, Mexico, Saturday, Dec. 29, 2018.

U.S. Customs and Border Protection encountered only six immigrants on the U.S-Mexico border in the first half of fiscal year 2018 whose names were on a federal government list of known or suspected terrorists, according to CBP data obtained by NBC News.

The low number contradicts statements by Trump administration officials, including White House press secretary Sarah Sanders, who said Friday that CBP stopped nearly 4,000 known or suspected terrorists from crossing the southern border in fiscal year 2018.

Secretary of Homeland Security Kirstjen Nielsen told reporters on Monday the exact number, which NBC News is first to report, was classified but that she was working on making it public.